This invention relates to magnetographic printing generally, and more particularly to non-impact electrostatic toner transfer techniques for both improving print quality and increasing print speed.
Often, when improvements are made in print quality, print speed (image transfer speed) is reduced, and when print speed is increased, print quality is reduced. For example, impact printing devices such as the electric typewriter or the so-called daisywheel type impact printer produce high quality print but are relatively slow. These devices print copy material by striking a preformed character against an inked ribbon onto paper. When using a carbon film type ribbon, these devices produce high quality print (i.e., letter-quality print as compared to print of lesser quality produced, for example, by dot matrix printers, thermal printers, computer line printers, and the like), but are generally noisy and print at the comparatively slow rate of 15-55 characters (symbols) per second. Certain other impact printers such as computer line printers are faster (e.e., they are capable of printing in excess of 900 lines per minute) but produce poor quality (i.e., non-letter-quality) print.
This print quality/print speed trade-off problem is also reflected in magnetic or magnetographic printing systems. Of those magnetographic printing systems which transfer images by impact, contact of the record medium (i.e., magnetic tape, drum, or other means carrying the toned magnetic image) with the paper introduces additional problems of contamination and degradation of the record medium. For example, in the system described in U.S. Pat. No. 3,735,416 issued to Ott, the useful life of the magnetic belt is shortened significantly due to toner particles becoming embedded in or otherwise contaminating the belt surface as a result of pressure of the record medium against the paper. This contamination causes excess toner to accumulate on the tape surface, soiling the non-image areas (background areas) of the record medium and, hence, the paper, producing a smudged or dirty background effect on the paper. As in Ott's system, the system described in the U.S. Pat. No. 3,740,265 issued to Springer transfers toner from a record medium (magnetic belt) to paper by means of pressure. This causes toner to become crushed into the surface of the belt, thus contaminating the belt and causing toner background to build-up, upon successive image transfers, in an unsightly manner on the paper. In U.S. Pat. No. 3,254,646 issued to Umera, a toned magnetic belt is struck by a hammer causing the belt to impact the paper for transfer of the toned image from the belt to the paper. In U.S. Pat. No. 3,749,833 issued to Rait, the transfer of toner particles to paper is accomplished by means of pressure rollers. Again, in both Umera and Rait, toner background problems would be encountered and belt or roller life would be shortened due to the toner particles becoming embedded in the surface of the magnetic medium.
Other magnetic or magnetographic systems attempt to avoid this problem of contamination of the record medium by maintaining an air gap between the record medium and the paper, and transferring the toner image electrostatically rather than by impact or applied pressure. One such system is described in U.S. Pat. No. 3,477,368 issued to Spaulding. Another such system is described in U.S. Pat. No. 4,175,265 issued to Nelson et al.
Generally, in both types of systems (impact and electrostatic transfer systems) the record medium and/or paper are moved relative to each other prior to or during transfer. However, we have found that as the speeds of relative approaching motion between the toned medium and the toner-receiving medium are increased to meet higher print speed requirements, the associated velocities of escaping air (aerodynamic forces) in the transfer region proximate to the record medium and the paper increase correspondingly, ultimately causing disruptive air currents to form in the transfer region and the toner present on either the medium or paper to disperse or spread. This problem is amplified where it is intended that there be area contact between the media, as opposed to line contact systems such as are characteristically embodied in electrophotographic copying systems. The spread or loss of toner is most visible at the edges of the character images. This toner disturbance produces poor character image edge acuity and unwanted background in, between, and around the character images. Further, the air currents increase and are cumulative to a maximum level at or proximate to the physical edges of the toned medium. Thus, those characters or protions thereof which are present near the edges of the medium tend to experience disproportionately greater disturbances than characters or portions thereof not so near the edges of the medium.
In some systems that use electrostatic transfer, a high voltage (for example, a voltage greater than 1000 volts) is found to be necessary for transfer of toner from the record medium to paper. To reduce this high voltage necessary during the transfer process, some systems teach the application of a form of pretransfer or bias voltage. However, this pretransfer or bias voltage, which is applied just prior to transfer, is intended to facilitate transfer of toner to paper and tends to weaken the bond between record medium and toner. A system of this type is described, for example, in U.S. Pat. No. 3,160,091 issued to Schwertz. Unfortunately, in such systems the likelihood of toner spread is also substantial, because the effect of the air currents upon the toner particles (i.e., the movement of air occasioned by the relative movement of the paper and the record medium) tends to increase the spread of toner as a result of the binding force between toner and latent image being weakened due to the applied bias voltage.
What is needed and would be useful, therefore, is a toner transfer method and system capable of providing high quality print without degrading image edge acuity or providing unwanted background effects when printing at high speed, and such is a principal object of this invention.